1. Field of the Invention
The present invention relates to a clock generator that is used in a video signal processing apparatus for processing a television signal conforming to the NTSC (National Television System Committee) and PAL (phase alternation by line) systems in order to generate a clock (hereinafter referred to as a pixel clock) for determining the rate at which picture elements (pixels) are scanned.
2. Description of the Prior Art
As a video signal processing apparatus, consider, for example, an apparatus for scanning an image. In such an apparatus, while an image is scanned, the image is broken up into a number of picture elements, and the signal values representing each picture element are extracted through a predetermined procedure. This type of video signal processing apparatus for scanning an image is provided with a clock generator, so that the signal values of each picture element are extracted in synchronism with a pixel clock generated by the clock generator. However, different television systems, such as the NTSC and PAL systems, require conformity to different standards, and naturally they require different clock frequencies for the pixel clock. For this reason, conventionally, a video signal processing apparatus needs to be provided with separate clock generators to generate different pixel clocks for the two television systems, even if the apparatus itself is designed to be compatible with both television systems.
In a clock generator compatible with the NTSC system, an oscillator uses a crystal resonator having a resonance frequency of, for example, 12.2727 MHz. On the other hand, in a clock generator compatible with the PAL system, an oscillator uses a crystal resonator having a resonance frequency of, for example, 14.7500 MHz.
In a clock generator compatible with the NTSC system, the oscillation frequency 12.2727 MHz equals 780 times the horizontal scanning frequency 15734.264 Hz of the NTSC system. This means that one horizontal scanning line is divided into 780 picture elements aligned horizontally. On the other hand, in a clock generator compatible with the PAL system, the oscillation frequency 14.7500 MHz equals 944 times the horizontal scanning frequency 15,625 Hz of the PAL system. This means that one horizontal scanning line is divided into 944 picture elements aligned horizontally.
However, these conventional clock generators require separate crystal resonators and separate oscillators for the NTSC and PAL systems. That is, conventional clock generators cannot achieve scanning for both of the NTSC and PAL systems unless they are provided with two crystal resonators at additional cost.